Many types of microcircuits are enclosed in electrically conductive, metallic or metallic-clad ceramic packages. Conductive packages are used because they provide a shield from ambient electrical noise that can cause the internal circuit to generate erroneous signals. The leads or pins which allow the electronic signals into and out of the package must be electrically isolated from the package itself. An insulator, typically of glass, between the lead and package, is one solution to the problem of isolation. Known in the industry as a glass-to-metal seal, the insulator can also provide a hermetic seal between the lead and package to prevent contamination of the internal circuit.
For several reasons, many glass-to-metal seals are constructed independently of the package and then permanently bonded into holes in the package walls. This independent device is called a feedthrough. In many cases, it is more efficient to bore holes in a package wall, insert feed throughs, and bond them into place than it is to design and make the complicated fixtures necessary to construct the glass-to-metal seal directly in the package. This is especially true where a relatively small number of the microcircuits are to be manufactured.
Another reason is weight. If it is desired to maintain the hermetic seal of the insulator in a wide range of ambient temperatures the package can be constructed from Kovar (a trademark of Carpenter Technology Corporation), which is an alloy that has a coefficient of expansion the same as that of heat-resistant glass, the typical insulating material. Thus, as the temperature rises, the glass insulator and Kovar package expand at the same rate maintaining the hermetic seal. The problems with Kovar housings include: 1)unavailability of custom sizes and 2) difficulty in machining, resulting in expensive setup and tooling charges compared to aluminum, even for standard size housings. Additionally, Kovar is heavy. In some applications, particularly aircraft- and spacecraft-related applications weight is of prime importancexe2x80x94consequently, a lowerweight material, such as aluminum is preferred for the package. However, aluminum has a coefficient of expansion greater than that of glass. So in order to provide hermetic seals in aluminum packages, feedthroughs are used and installed through the use of solderxcx9c or epoxy.
The feedthrough consists generally of a conductive ferrule, a conductive lead extending through the ferrule and spaced from the ferrule edges, and an insulator, typically heat-resistant glass, holding the lead in places isolating it from the ferrule, and providing a hermetic seal. There are two basic types of feedthroughs, matched and compression a matched feedthrough, the materials used for the ferrule, insulator, and lead have essentially the same coefficient of expansion. This keeps the hermetic seal intact during temperature changes, both during manufacture and in use. In the compression seal, the coefficient of expansion of the ferrule is greater than that of the insulator, so that as the feedthrough cools during manufacture, it contracts more than the insulator, compressing the insulator against the lead.
Leads can exit vertically through the floor of the microcircuit package. Vertical leads can be plugged into sockets, printed circuit boards, or other connecting devices. They can be soldered, clamped, or otherwise hardwired into vias on the circuit board. They can also be bent at right angles to extend out from under the package and soldered to pads or vias on circuit boards, for example, when using surface mount technology (SMT). The major disadvantage to this use is that, unless the leads extend vertically a short distance before being bent outwardly, they will make electrical contact with the package edge, causing a short circuit that prevents the microcircuit from operating properly. If the vertical portion is not long enough, the lead can be short-circuited to the package by the solder that connects the lead to the circuit board. If the vertical portion is too long, the package may stand too high on the circuit board or may be susceptible to being bend over, which can also cause short circuits.
Alternatively, leads can exit horizontally through the side walls of the microcircuit package. Leads that exit horizontally can also be plugged into sockets, but are normally provided for SMT. When used for SMT, the leads extend horizontally from the package a short distance so they do not short against the package, are bent downwardly to the circuit board, and then are bent outwardly along the circuit board. One disadvantage is that the leads must be bent, which is an extra step in the manufacturing process. The major disadvantage, however, is that these leads use up a significant amount of valuable circuit board area. The amount that the lead must extend horizontally from the package before being bent downwardly increases the circuit board surface area needed to accommodate the package, meaning that the circuit board must be larger to provide the same functionality. Additionally, the exposed horizontal lead causes unwanted inductance in high frequency and wide band circuits. Also, the horizontal lead functions as an antenna that transmits energy, affecting other components.
The current solution to the SMT problem is to manufacture right-angle glass-to-metal seals directly in the package. The right-angle lead exits the package through a notch in the edge of the package so that the lead is flush with the circuit board. However, the manufacturing efficiency and package weight problems associated with these direct glass-to-metal seals described above remain.
An object of the present invention is to provide a microcircuit package feedthrough that can be used with surface mount technology.
Another object is to provide a microcircuit package feedthrough that significantly reduces the possibility of a short circuit between the lead and package when used with SMT.
Yet another object is to provide a microcircuit package feedthrough that uses less circuit board area for SMT than conventional feedthroughs of the present art.
A further object is to provide a microcircuit package feedthrough that requires fewer manufacturing steps to install than conventional feedthroughs.
Another object is feedthrough cost effectiveness, compared to other designs.
Another object of the present invention is that it allows circuits to be used with SMT, otherwise unavailable in circuits incorporating kovar package housings, inter digital, combine, high frequency, wide band cavity and ceramic circuits.
Another object is that the feedthrough dimensions allow a 50 ohm impedance to be provided DC to 16 gigahert, compared to full packages limited to 1 GHz.
Other objects of the present invention will become apparent in light of the following drawings and detailed description of the invention.